Using
St. Astier NHL plastering mortars instead of non hydraulic putty mortars
reduces the working time by about 50%. NHL mortars offer similar vapor exchange
qualities as putty mortars but are more robust, can be sprayed and used for decorative
plasterwork without the addition of gypsum. Requiring less after care than putty,
it can be applied in 2 coats on good level backgrounds.

Mortar.
Plastering in hydraulic lime mortar normally consists of two or three-coat work. Lime
plaster made with feebly or moderately hydraulic lime and sand is the basis for
this guide. This type of lime sets and hardens predominantly by an hydraulic set
and re-absorption of Carbon Dioxide from the air. By its nature the drying and
absorption process is slower than gypsum plasters, therefore lime plaster curing
should not be hurried allowing approximately 3-5 days per coat depending on the
hydraulic lime used.

Background. For plastering
on the hard, the background will normally be brick or stone. The surface should
be clean, free from dust and any organic materials such as lichens etc. Test the
surface of masonry backgrounds for dust by applying a piece of masking tape to
the background and immediately remove, examine the sticky side for traces materials
that may affect the bond between the plaster and the wall.

Internal walls
can be uneven and rough, often with areas that have been altered. Different background
conditions are therefore common and this needs to be addressed before plastering. Deep
holes, wide joints or pockets should be dubbed out in thin layers of mortar with
pinnings tightly bedded in mortar, keyed and left to cure. The aim of preparing
the background should be to achieve a surface that can take a first coat of consistent
thickness, and to provide an adequate key for this first coat. The quality of
preparation work is vital to the quality of the finished job. Suction between
the first coat and the wall (and between all subsequent coats) is the primary
means of bonding although a physical bond is also important. Different materials
have different levels of suction, so for instance where a door way has been knocked
through a stone wall and the edges built in brick, the brick may well have a different
level of suction to the stone. Understanding and controlling suction is important
for successful work.

For wood lath and plaster work, laths should be fixed
by butt and break joints to joists or battens securely fixed back to wall or ceiling,
with gaps between the laths of approx, 8 - 10mm . The support battens or ceiling
joists should be spaced so that the lath does not give unduly in the centre. Wide
spacing of battens or joists may require intermediate support or thicker laths.
Sawn or riven laths (traditionally hand made) should be thoroughly damp before
fixing. Dry laths swell when wet mortar is applied to them, sometimes causing
the laths to bow in or out. Nails for fixing laths should be thin shank to avoid
splitting the ends. Building paper and insulation is occasionally placed between
laths and outside walls to comply with current building control requirements,
this will have an effect on the drying rate and prevent proper rivet formation
when fixed hard against the back of the lath. If building paper and insulation
are essential, use moderately (NHL 3.5) or eminently hydraulic lime (NHL 5) for
the first coat as they have faster natural sets, maintaining at least a 20mm gap
between the paper and the lath.

First coat or Render coat. Rendering stuff
is made with NHL 3.5 or NHL 2 in a 1:2 ratio with well graded sharp sand
3.5mm down. Rendering stuff is mixed before use as described in the Making
NHL Mortars data sheet. Hair or fibre reinforcement (if desired,
but not necessary with hydraulic limes, except on lath work) is teased into
the mortar and repeatedly chopped and turned until a good even distribution
is achieved. The hair should be long and strong, free from lumps and clumps. Fibre
reinforcment should be alkali resistant. It should be abundantly visible
as a beard around the edge of an inverted trowel full of mortar.

When working on the hard the first coat of mortar
should be applied by throwing (a spray gun can be used) or with a laying-on trowel
on to a dampened but not wet background at approximately 9 - 12mm thick (generally
hair or reinforcing is not required on the hard). The render coat should not be
straightened.

For application onto laths, mortar should be trowel applied
as evenly as possible and pressed home to form rivets between and behind the laths.

The coat should cover the lath by approximately 8-10mm. Any initial shrinkage
that takes place in the drying out phase, should be lightly floated back.
Tighten the entire surface in with a cross grained wood float, closing back by
further dampening and tightening in a close circling motion. Key the surface with
a comb, or with a lath scratcher for plaster on laths, taking care not too score
to deeply. In the case of lath work ensure that scratching is across, not in line
with the lath. Thereafter if necessary control the rate of drying out by misting
the surface with clean water or lime water, until all shrinkage has stopped and
the mortar has hardened sufficiently to receive the second coat. Do not over-wet
the surface (if water droplets appear on the surface it is over-wet). Plastering
on laths may take longer to dry as there is no suction of moisture into the background.

Second
coat orFloating coat (this is the straightening
coat). Using the same mortar as the render coat and following the same method
of mixing, apply with a laying-on trowel to a dampened background. At this stage,
if straightening is necessary, fill out irregularities in thin layers and apply
the floating coat to the desired line using rules or dabs. Maximum thickness
should be 12- 16mm in one pass. Tighten in as before with a cross grained wood
float and key lightly to receive the finish. Any irregularities in the background
must be made good at this stage as it will not be possible to straighten the finishing
coat due to its thinness.

Setting
Stuff or Finishing coat (hair is not required). The finishing
coat is made with NHL2 in a 1:1 with fine silica sand 0.8 to 0.075mm, clean and
free from silt or clays. Better finishing will be achieved by re-working the setting
stuff the day after. Apply with a laying-on trowel in two passes to an overall
thickness of 2mm approx. Tighten in with a cross grained wooden float, dampening
as necessary as the set takes up. Setting stuff sets by suction from the background. The
final finish is produced by trowelling the surface with a steel float and dampening
as necessary. Finish in one direction, usually top to bottom. Patching finished
lime plaster into a repair requires care. The finish is sand based and will abrade
and degrade existing edges leading to a dull surface around the repair. A clean
break of a few mm is necessary around the patch and this can be filled later with
NHL 2 made into a putty (no sand added) when the patch is fully hardened.

Decoration: wallpaper or paint finishes should only be applied to fully
dry and set work. Lime paint, lime wash, distemper and casein paints are traditional
finishes for new work.

Protection. Work should never be undertaken in frosty conditions
or where the temperature is likely to fall below 40oF during
the execution of the work or until the mortar has hardened. Protection should
remain in place for as long as possible (see data sheets on setting / curing
times for individual limes). Ensure that the rate of drying is consistent
and that strong draughts are excluded from the working area. This is particularly
important where a building has windows removed or doors open. Never force
the drying by introducing forced or excessiveheating. If heating
is required to maintain a proper working temperature use propane heating,
this has the effect of producing both moisture and heat simultaneously.
Ensure the temperature is adequately controlled.

Good working practices.
Adequate preparation and protection are essential. Due care and attention
should be applied to all work. Sample panels should always be done, allow sufficient
time.

This
document is a guide only and is not intended to be a specification. Its purpose
is to provide the reader with helpful information that may assist in determining
the correct choice of materials, methods of application and the best working practice.
The guidelines refer to our experience with St. Astier NHL binders and some recommendations
might not be applicable to other products.